This invention especially relates to improvements in the production of 2,3,3,3-tetrafluoro-2-propene, also known as 1234yf, and having the chemical formula, CF3—CF═CH2.
This chemical compound has zero ozone depletion potential and low global-warming potential such that it may be useful and desirable as a replacement for existing materials used in refrigeration, foam blowing and other applications where fluorocarbons such as 1,1,1,2-tetrafluoroethane, also known as 134a, and known also by the chemical formula, CH2F—CF3, are currently utilized.
It is known in the art that 1234yf is produced from 1,1,2,3-tetrachloropropene (TCP or CCl2═CCl—CH2Cl) using a non-integrated three step route; (see for example U.S. Pat. No. 8,084,653, the disclosure of which is hereby incorporated herein by reference):                Step (1); TCP+3HF→1233xf+3HCl (where 1233xf is CH2═CCl—CF3)        Step (2); 1233xf+HF→244bb (where 244bb is CF3—CFCl—CH3)        Step (3); 244bb→1234yf+HClHydrofluorination reactions, like that of Step (2) above, may be conducted in the liquid phase. In a commercial process, the objective is to conduct the conversion of 1233xf to 244bb with a conversion of greater than about 90% and with a selectivity of greater than about 90%. Since this is a heterogeneous reaction containing immiscible liquids and/or some solid catalyst material, in order to maximize the conversion of 1233xf to 244bb, it is important that the reactants and catalyst be uniformly mixed. One of the problems encountered and to overcome is channeling which creates potential eddy currents in addition to substantially dead spaces, all of which may lead to nonhomogeneous material treatment, thereby reducing the conversion of 1233xf and selectivity to 244bb.        
The present inventors have developed an apparatus for use in liquid hydrofluorination reaction which tends to minimize the channeling effect and promote the conversion of a fluoroolefin to a fluorocarbon, for example, the formation of 244bb from 1233xf.
However, the conversion of the fluoroolefin is just one aspect of the problem. At the same time, the objective is to maximize the selectivity to 244bb by minimizing side reactions from the hydrofluorination reaction, such as overfluorination. For example, if the fluorocarbon produced has a suitable leaving group thereon, such as a chlorine atom, the fluorocarbon may undergo a subsequent substitution reaction with HF in which the fluorine atom substitutes for the leaving group. For example, in the hydrofluorination of 1233xf to form 244bb, the reaction of 244bb with a second molecule of HF to form 245cb, as indicated below, is a side reaction: 244bb+HF→HCl+245cb (where 245cb is CF3CF2CH3). Further, the reaction of 1233xf with 2 moles of HF less to the formation of 245cb: 1233xf+24F→HCl+245cb
Unless the reaction conditions can be adequately controlled, the formation of the second product from the reaction with hydrogen fluoride may not only decrease the yield of the 244bb fluorocarbon, but also decrease the selectivity to the 244bb fluorocarbon, i.e., it may instead increasingly promote the formation of the side reaction product. The present inventors have found a method which promotes only the hydrofluorination reaction to occur and minimizes the formation of the side products.
In addition to the channeling and the side reaction issues, another problem encountered in the liquid hydrofluorination process is the use of corrosive material. The hydrofluorination reaction uses and generates corrosive compounds, such as, for example, hydrogen fluoride and chlorine gas, the latter of which can be used to regenerate the hydrofluorination catalysts. Both tend to corrode the reactor in which the reaction is conducted, even reactors comprised of corrosion-resistant materials such as Inconel 600, NAR25-50MII, Hastelloy C, Hastelloy G-30, duplex stainless steel, and Hastelloy C-22. Corrosion of the reactor compromises the structural integrity of the reactor and reduces its useful life. Therefore, a need to minimize reactor corrosion exists.
Generally, reactors having a molded liner, such as a rotary-baked or sprayed-on liner, are not suitable for large-scale commercial production reactions. Reactors having such liners must be baked in large kilns or ovens, which are expensive and frequently unavailable. Indeed, fitting a large reactor, for example, greater than about 1,000 gallons, with a baked liner is impractical.
A molded liner not only imposes practical limitations on the size of the reactor, but also introduces additional structural limitations. It has been found that molded liners often tend to be permeable and, under high pressures and over time, reactants tend to penetrate the liner and develop pressure between the liner and the reactor wall. This pressure causes the liner to blister, and eventually the liner comes loose. The problem of liner penetration is exacerbated by the absence of weep holes in a molded-liner reactor. Ordinarily, weep holes allow reactants that penetrate the liner to escape from the reactor. A molded liner, however, generally cannot be used in a reactor with weep holes. When applying a molded liner, a fluid fluoropolymer is applied to the reactor wall, and, thus, holes in the reactor wall will result in holes in the molded liner. Holes in the liner obviously compromise the reactor's ability to be pressurized. Therefore, while a rotary-baked, fluorine-resin liner may minimize reactor corrosion, its structural limitations nevertheless limit the reactor's size and/or useful life.
Therefore, a need exists for a commercially viable method of producing a wide range of HFCs while minimizing reactor corrosion. The present invention fulfills this need among others.